Question composing a variable color photo output as a grey scale phone

I have a client with a very large library of headshots in color. They would like to use these same photos in variable web store documents and have the preview show them as grey scale like they would be printed on our black only lasers.

I know this is a Macrom related question, but I believe this has to be done template side and not at the store level.

The flip side of this is that the client would either have to trust the grey scaling will be ok for each order, or have somebody create grey scale photos for this specific project.

Thank you for any and all possible help.